Bistable and other nonlinear optical devices have been known for some time, and a wide variety of signal processing functions can be carried out by means of bistable devices. "Bistable" and "nonlinear" are used herein interchangeably unless indicated otherwise by the context. For instance, U.S. Pat. No. 4,012,699 discloses, inter alia, amplification of a light beam by means of a cavity-enclosed positive-temperature nonlinear medium. A recent monograph, H. M. Gibbs, Optical Bistability: Controlling Light With Light, Academic Press (1985) can serve as an introduction to the field and discusses many of the possible modes of operation of bistable optical devices. For instance, on pages 1-17, incorporated herein by reference, are given brief discussions of bistable optical logic devices (both two-state and many-state), of an optical transistor, of optical discriminators, limiters, pulse compressors, oscillators, gates, and flip-flops. And pages 195-239, also incorporated herein by reference, contain a detailed discussion of optical switching.
Many of the nonlinear optical devices comprise a nonlinear Fabry-Perot (FP) etalon, a fixed-spacing optical cavity with an optically nonlinear medium within the cavity. Furthermore, much of the work on optically nonlinear devices has focused on devices using solid, typically semiconductor, mostly GaAs-based, nonlinear media. Such media are, for instance, homogeneous GaAs, and GaAs-AlGaAs multiple quantum well (MQW) structures.
As described for instance by J. L. Jewell et al, Materials Letters, Vol. 1(5-6), pp. 148-151 (1983), GaAs-based nonlinear FP etalons are fabricated by a rather difficult process. The process exemplarily involves deposition of an about 3 .mu.m GaAs-AlGaAs MQW structure on a GaAs substrate by molecular beam epitaxy, and removal of the substrate by grinding and selective etching so as to leave the MQW structure. The resulting 3 .mu.m flake can then be mounted between dielectric mirrors, thereby producing a nonlinear FP etalon.
FIGS. 1 and 2 schematically show another prior art FP etalon formed by etching away a portion of a GaAs substrate. See, H. M. Gibbs et al., Optics News, Vol. 5(3), pp. 6-12 (1979), incorporated herein by reference. The etalon 12 was produced by depositing a 0.2 .mu.m Al.sub.0.42 Ga.sub.0.58 As etch stop layer 15 onto the 150 .mu.m GaAs substrate 10, followed by deposition of the 4.1 .mu.m GaAs active spacer layer 14 and of a further AlGaAs layer 15. After grinding and etching away of the GaAs substrate in a 1 mm.sup.2 region 11 of FIG. 1, a (non-active) multilayer mirror 16 was formed by vapor deposition on each layer 15.
Karpushko et al. (Journal of Applied Spectroscopy USSR, Vol. 29, p. 1323 (1978)) disclosed an optical interference filter comprising two mirrors with a ZnS spacer therebetween, that exhibited optical bistability.
Prior art nonlinear etalons as described above have significant shortcomings. Among these is the difficulty of controlling the etching of the substrate sufficiently well to achieve the high thickness uniformity required for high-finesse mirrors. As is well known, if F is the desired finesse associated with the etalon, then the thickness of the spacer layer/etch stop layer combination has to be uniform to at least .lambda..sub.o /2nF over a significant portion of the flake, where .lambda..sub.o is the vacuum wavelength of the operating radiation of the device, and n is the refractive index of the material. For instance, if a finesse of 10 is desired, the spacer thickness has to be constant to within about 11 nm for .lambda..sub.o of about 0.87 .mu.m. In etalons of the type described by Jewell et al. (op cit), it is also difficult to achieve good and stable mechanical and thermal contact between the flake and the mirrors. Furthermore, the prior art methods for producing nonlinear FP etalons tend to require substantial skill and manual dexterity, and do not lend themselves to the formation of multi-etalon arrays.
Due to the promise held by nonlinear FP etalons, for instance, in the field of optical data processing, including optical computing, and in optical communications, it would be highly desirable to have available a method for producing such etalosn that is not subject to these and other shortcomings of the prior art. This application discloses such a technique. For information on optical computing, see Proceedings of the IEEE, Vol. 72(7) 1984, especially A. A. Sawchuck et al (pp. 758-779), and A. Huang (pp. 780-786). A. Huange et al., Proceedings of the IEEE Global Telecommunications Conference, Atlanta, Ga., 1984, pp. 121-125 discloses telecommunications apparatus that can be implemented using nonlinear optical devices according to the invention.